Opacity refers to the degree to which a paper will transmit light, expressed in percent. A high degree of opacity prevents shining through of the print. The higher the opacity value, the more opaque the paper.
The rough surface of paper is evened out in a smoothing process. Irregularities have a negative effect on paper quality.
Yellowing of paper results from exposure to daylight and increased temperatures. The degree of yellowing is expressed in a yellowing index.
Fillers / Binders / Additives
Fillers improve printability, as they fill the gaps between the fibres. In paper production, natural mineral, inorganic substances such as calcium carbonate (chalk), kaolin (china clay), talcum, and also synthetically produced substances such as titan dioxide are used. Kaolin is also added to increase glossiness.
Fillers improve surface smoothness, calendering, opacity, brightness and softness. Calcium carbonate adds to the durability of a paper. Filler content may account for up to 30% of a paper's weight.
The fixing of the fillers is done using a retention agent and cationic starch won from maize or potatoes. The produced raw paper consists on average, considering all types of paper, to 4/5 of fibres and to 1/5 of natural fillers.
Optical brighteners and glues are considered additives. Glue regulates the absorbency of paper and is added to the paper mass to make the paper writable and to prevent the colours from blurring. Blurring of ink occurs in papers without glue, such as blotting paper for example.
In the field of printing, the vibrancy of colour images depends on the brightness of the paper used. The higher the brightness of the paper, the higher the quality of the paper within its category. Optical brighteners may also be used.
Brightness is expressed in percent and measured using a brightness tester.
Roughness is expressed in micro metre (µm) and measured with a roughness tester. The closer the measured surface approaches the ideal surface (glass pane), the lower the measured value and the smoother the paper.
The kappa number indicates the residual lignin in pulp. The lower the kappa number, the lower the lignin content. Low lignin content ensures greater age resistance of the paper.
Age resistant papers consist of primary fibres (bleached pulp) and show a high pH value (approx. 8). For graphical papers standards have been defined with regard to age resistance, including limit values for the kappa number (<5). The pH value is set in the vat.
Voluminous papers are softer and show lower tensile strength, as they are thicker than others that have the same weight, and thus have a lower apparent density (<1.5). The volume is calculated based on the thickness and grammage.
Stiffness is an important benchmark of paper. It is the resistance to bending and its ability to support its own weight when handled. The degree of elasticity is mainly determined by the selection of the raw material and the refining of the pulp.
Measurements are carried out according to international standards, which demand 50% humidity and 23°C room temperature prior and during testing.
Porosity depends on the type of pulp used, the refining of the raw material, the basis weight of the paper, as well as on the use of fillers. In general, every paper has air passages. Porosity expresses the extent to which a paper surface will allow the penetration of air and is measured with a porosity tester. The sheet of paper is placed into the tester which presses 100ml of air through a test area of 6.42cm² with 1.23 kPA and measures the time needed for this process.
The ash content describes the residue left after complete combustion of paper and is expressed in percent. The higher the filler content, the higher the ash content of a paper.